neutrino

The three types of neutrinos show in relation
to the other known elementary particles. Credit: Fermilab.

The neutrino is a subatomic particle with no charge and very little mass, that interacts
only by the weak force and by gravity.
It is a member of the lepton (lightweight)
family of particles to which the electron also belongs. Neutrinos travel at or close to the speed
of light and have zero electric charge. Ghostlike in their ability to
avoid interacting with matter, it has been estimated that neutrinos could
pass through 100 light-years of solid lead with only a 50 percent chance
of being absorbed. The name, which means "little neutral one" in Italian,
was coined by Enrico Fermi in 1933.

The existence of neutrinos was first postulated in 1930 by Wolfgang Pauli to ensure conservation of energy and angular momentum in beta
decay; effectively, it carries away excess energy in nuclear reactions.
Three different types of neutrinos exist, known as electron-, mu-, and tau-neutrinos,
corresponding to the three massive leptons: electrons, muons,
and tau leptons.

The Sun produces neutrinos from thermonuclear fusion reactions in its core and, since these
neutrinos pass clean through the Sun and then all the way to Earth, they
provide a way of glimpsing into the heart of a star. A large flux of neutrinos
carries away most of the energy of a supernova and neutrinos are one of the candidates for dark
matter. So, neutrino astronomy offers an important new window on the
universe beyond the electromagnetic spectrum. Because neutrinos pass so
easily through matter, they're very hard to detect: large masses of stopping
material and indirect detection of the effects of neutrino absorption are
needed. Among the most powerful neutrino "telescopes" are the Sudbury
Neutrino Observatory in Canada and the Super-Kamiokande in Japan.

One of the great puzzles of astrophysics in recent decades, has been the
discrepancy between the number of neutrinos detected coming from the Sun
and the number expected from theory. The so-called solar neutrino problem,
which emerged from measurements by Ray Davis and his pioneering neutrino detector in a South Dakotan gold mine, suggesting
that only one-third the expected number of solar neutrinos were arriving
at Earth, has now been effectively cleared up by recent data from the Canadian
and Japanese instruments. These data show that some of the electron-neutrinos
produced in the Sun's core change into the other types of neutrino while
en route to Earth. Earlier experiments, including that of Davis, only registered
the electron-neutrinos and therefore suggested a shortfall. The newer experiments,
such as that at Sudbury, pick up all the varieties of neutrino and have
shown that the total count of solar neutrinos is in line with the rate of
electron-neutrino predicted by orthodox theory of nuclear reactions inside
the Sun.